Ferroelectric Memoirs
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Ferroelectric Memoirs J. F. Scott Department of Physics, Cavendish Laboratory, Cambridge University, Cambridge, U. K. ABSTRACT A personal view of ferroelectrics and multiferroics research is given with suggestions for future work, emphasizing nano-dynamics. INTRODUCTION Retrospective: I began my work on perovskite oxides in 1968 with a study of strontium titanate [1]. In this study we found that the structure below ca. 110K that had been determined by X-ray techniques [2] was incorrect, and that the EPR results of Unoki and Sakudo [3] which were new but generally unnoticed by the condensed matter community were correct. This and the work on lanthanum aluminate that followed shortly after [4] were the first studies in which the advantage of Raman spectroscopy over XRD was demonstrated for structural determinations; in systems in which only light ions such as oxygen or fluorine move, and in which the distortions are very small nearly rigid rotations, XRD is not the best tool. This continues to the present, with recent elegant studies via neutron spectroscopy showing that X-ray studies of bismuth ferrite structures were incorrect [5,6]. Somewhat surprisingly the study of strontium titanate at low temperatures remains very fruitful, with Physical Review Letters this year establishing that the lowtemperature phase of oxygen-18 samples is probably triclinic [7,8], in accord with the NMR results of Blinc’s group [9,10]. and that the claim that SrTiO3 exhibits second sound [11.12] now seems quite unlikely, and that flexoelectricity plays a key role in the low-T spectroscopy [13,14]. By 1980 my work at Colorado had shifted to multiferroics [15,16], where the family of BaMnF4, BaCoF4, and BaNiF4 seemed important, based upon their very early studies in Moscow [17.18] . However, a rather odd decision terminated that work: When my NSF research grant on ferroelectrics was renewed, I was required by the NSF monitor to delete all work on multiferroics and magnetoelectrics, based upon one strongly negative referee evaluation that said “Work on magnetoelectrics has no future or scientific interest.” My contract monitor said that this report came from “the best authority on ferroelectrics at the most prominent US university laboratory for such work.” I have always assumed that was Penn State. So magnetoelectrics were left to Hans Schmid and the boracites for the time being. There is probably a moral here. My students and I did achieve two things on this topic before being told to discontinue the work: (1) we showed that ferroelectricity could actually cause weak ferromagnetism by tilting antiferroelectrically ordered spins [19,20]; and (2) we showed that trilinear coupling of polarization P, magnetization M, and sublattice magnetization L, could cause a symmetry change. This was recently rediscovered by Benedek and Fennie and by Ghosez and Triscone in a more general context [21,22]. At present multiferroics have become a cottage industry, with
significant progress on voltage-controlled magnetic tunnel junctions [23] and room-tempera
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